1. Field of the Invention
This invention relates to a fuel supply system for use with internal combustion engines and, more particularly, to a fuel supply system in which assist air is supplied to a fuel injection valve to improve atomization of fuel.
2. Description of the Related Art
A fuel supply system is known in which a fuel injection valve is provided in an intake pipe on the upstream side of an intake port of an internal combustion engine, and an air-fuel mixture in which a fuel from the fuel injection valve and air are preliminarily mixed is supplied to the engine. Pulverization or atomization of the injected fuel and the atomized state of the fuel are controlled so as to improve combustion characteristics.
If a combustion chamber is provided with two intake valves for the purpose of achieving high-speed and high-power performance of an internal combustion engine, it is necessary to evenly distribute fuel from a fuel injection valve in the vicinity of an intake port to the two intake valves. To achieve this effect, a fuel injection valve has been proposed which is designed so as to form bundles of fuel spray by separating fuel flow in two directions from the fuel injection valve (Japanese Patent Unexamined Publication No. 4-50472). In this case, fuel is jetted from two injection holes so that two fuel flows are evenly directed toward the two intake valves. Simultaneously, air and fuel are caused to impinge against and mix with each other so that injected fuel is finely pulverized.
For example, in the case where, as shown in FIG. 3, fuel is to be jetted from a fuel injection valve 1 toward two intake valves 101 and 102 of a combustion chamber, fuel pulverization/atomization characteristics cannot be improved sufficiently, although fuel jetted from the injection holes can be separated into two flows.
The inventors of the present invention has experimentally found the following fact. If a main air hole is provided at a position between two injection holes for separating a fuel spray in the fuel injection system disclosed in Japanese Patent Unexamined Publication No.4-50472, it is necessary to supply air through the main air hole at a very high flow rate in order to separate a fuel spray in two directions. For example, fuel jets from the two injection holes cannot be separated suitably unless the area of a sub air hole is made smaller than that of the main hole so that the area ratio (main air hole area)/(sub air hole area) is about 6. With respect to non-supercharging introduction of air from the upstream side of an intake throttle valve to a fuel injection valve in an internal combustion engine, there is a limit to the maximum air flow rate in view of idling, at which air can be introduced into the fuel injection valve, for a reason relating to the internal combustion engine system. For example, the maximum air flow rate is limited to 1.3 m.sup.3 /h. Therefore, if the main air hole and the fuel injection holes are spaced apart from each other as in this fuel injection system, the air flow cannot be utilized effectively and it is difficult to suitably pulverize or atomize fuel.